Narrowband light (e.g., light containing only a small range of wavelengths such as a laser light) in an optical system may be too bright (e.g., high amplitude) for a detector, or may need to be reduced without affecting nearby wavelengths of interest. It is therefore desirable to control the amplitude of the narrowband light.
Conventional methods for controlling the amplitude of a narrowband light involve the use of a fixed filter. Conventional fixed filters comprise a thin-film coating on a glass substrate with some fixed transmission spectrum. Fixed filters, however, require knowledge of the wavelength characteristics of the light of interest in advance of manufacture. Additionally, fixed filters are designed and manufactured for specific wavelengths and are therefore incapable of filtering other wavelengths. Accordingly, when the application requirements or wavelengths of interest change, a replacement fixed filter must be designed and manufactured. Furthermore, the sharpness of the transmission cut-offs and cut-ons (i.e., how sharply the filter switches between high-transmission and low-transmission versus wavelength) may be limited by conventional thin-film coating design constraints. Moreover, thin-film coating designs which use multiple coating layers increase the probability that the fixed filter will suffer from common coating manufacturing errors, which may further limit the actual bandpass of the fixed filter and the sharpness of the cut-ons and cut-offs.
According to another conventional method for controlling the amplitude of the narrowband light, a fixed filter may be made to be a tunable filter by tilting the fixed filter at some angle, thereby shifting its transmission spectrum by a limited amount. This method, however, involves mechanical motion, which decreases reliability of these optical systems due to the presence of moving parts. Since such moving parts are susceptible to failure, there is also some decrease in ruggedness in applying this method. Additionally, this method requires electrical power to activate any motors or solenoids responsible for tilting the fixed filter. Furthermore, tilting of the fixed filter to a desired position inevitably requires some time to accomplish. Finally, the magnitude and direction of the spectrum shift is limited.
According to another conventional method for controlling the amplitude of the narrowband light, a pair of polarizing filters configured to rotate relative to each other may be utilized. This method, however, changes the transmission over a wide waveband with no differentiation between nearby wavelengths. Further, this method also involves moving parts, thereby decreasing the reliability of these systems as described above.
According to another conventional method for controlling the amplitude of the narrowband light, multiple fixed filters may be switched into the path of the incoming light one at a time by the use of some arrangement of filters on a controllable wheel or other filter holder. This method, however, occupies a large amount of volume due to the number of fixed filters required, the hardware necessary to mount and switch the fixed filters, and the extra space required to move the filters around each other. Additionally, the variability in the filter functionality that this method provides is limited by the finite number of individual fixed filters. Furthermore, this method also involves moving parts, thereby decreasing the reliability of these systems as described above. Additionally, this method requires electrical power to activate any motors or solenoids responsible for switching the fixed filters into and out of the incoming light path. Furthermore, such switching to a desired position inevitably requires some time to accomplish.